What is in this article?:
• Irrigated beans were particularly hard hit because they were up and growing a little faster than the dryland beans.
• The big problem caused by the Dectes stem borer was more from lodging, caused by the weakened stems, than from actual damage to the beans, though that was bad enough.
A RARE PEST, Dectes stem borer, cut yields for award winning South Carolina soybean grower Jason Carter.
15-20 percent yield loss
“I cut open the soybean stems in the trouble spots in the field and the whole inside of the stem was gone. It was amazing to me that the soybeans even survived, much less produced any beans. I have no doubt my yields were cut by 15-20 percent — maybe more in some spots,” he adds.
Irrigated beans were particularly hard hit because they were up and growing a little faster than the dryland beans. The big problem was more from lodging, caused by the weakened stems, than from actual damage to the beans, though that was bad enough, Carter adds.
University of Kentucky Entomologist Doug Johnson has likely had as much experience with Dectes stem borers as anyone in the South.
He says the insect showed up sporadically a decade or more ago in some parts of Kentucky, but has become more troublesome the past few years in traditionally high soybean producing areas.
How widespread the problem is in the Southeast is unclear, primarily because of the rare occurrence of the pest and the likelihood that its damage is attributed to other causes.
No one, including Johnson, is predicting the pest will become a major threat to soybeans in the Southeast, but it is one growers need to know about and scout for in their fields.
Johnson says the Dectes (or soybean) stem borer is a small (3/8-inch long beetle. The adult beetle is pale gray and has prominent black and gray banded antennae that are as long as or longer than the body.
Single eggs are deposited in cavities that female beetles chew into leaf petioles or stems. If eggs are laid in leaf petioles, larvae will feed in the petiole for several days before tunneling into the stem.
The trifoliate leaf and the petiole then wilts, dries up and drops from the plant.
The University of Kentucky entomologist says dead leaves can be observed in the canopy for a number of days. For Jason Carter this stage of development was the first time he noticed damage in his beans, though most of the damage was already done by the point.
Johnson says Dectes larvae are legless with small, brown heads. Their bodies are deeply segmented in an accordion-like fashion and conspicuously enlarged near the head with the body gradually tapering toward the rear end. Fully-grown larvae are creamy white and one half to five eighths inch long.
Research in several states indicates soybean stem borer over-winters as a larva in the base of hollow, girdled stems.
Carter says this concurs with his production practice. He has strip-tilled all his land for the past 15 years. In addition, the winter of 2011-2012 was unusually warm, which likely contributed to the problem.